This invention relates to sound recording and reproduction systems, and is particularly concerned with stereo sound reproduction systems wherein at least two loudspeakers are employed.
It is possible to give a listener the impression that there is a sound source, referred to as a virtual sound source, at a given position in space provided that the sound pressures that are reproduced at the listener""s ears are the same as the sound pressures that would have been produced at the listener""s ears by a real source at the desired position of the virtual source. This attempt to deceive the human hearing can be implemented by using either headphones or loudspeakers. Both methods have their advantages and drawbacks.
Using headphones, no processing of the desired signals is necessary irrespective of the acoustic environment in which they are used. However, headphone reproduction of binaural material often suffers from xe2x80x98in-the-headxe2x80x99 localisation of certain sound sources, and poor localisation of frontal and rear sources. It is generally very difficult to give the listener the impression that the virtual sound source is truly external, i.e. xe2x80x98outside the headxe2x80x99.
Using loudspeakers, it is not difficult to make the virtual sound source appear to be truly external. However, it is necessary to use relatively sophisticated digital signal processing in order to obtain the desired effect, and the perceived quality of the virtual source depends on both the properties (characteristics) of the loudspeakers and to some extent the acoustic environment.
Using two loudspeakers, two desired signals can be reproduced with great accuracy at two points in space. When these two points are chosen to coincide with the positions of the ears of a listener, it is possible to provide very convincing sound images for that listener. This method has been implemented by a number of different systems which have all used widely spaced loudspeaker arrangements spanning typically 60 degrees as seen by the listener. A fundamental problem that one faces when using such a loudspeaker arrangement is that convincing virtual images are only experienced within a very confined spatial region or xe2x80x98bubblexe2x80x99 surrounding the listener""s head. If the head moves more than a few centimetres to the side, the illusion created by the virtual source image breaks down completely. Thus, virtual source imaging using two widely spaced loudspeakers is not very robust with respect to head movement.
We have discovered, somewhat surprisingly, that a virtual sound source imaging form of sound reproduction system using two closely spaced loudspeakers can be extremely robust with respect to head movement. The size of the xe2x80x98bubblexe2x80x99 around the listener""s head is increased significantly without any noticeable reduction in performance. In addition, the close loudspeaker arrangement also makes it possible to include the two loudspeakers in a single cabinet.
From time to time herein, the present invention is conveniently referred to as a xe2x80x98stereo dipolexe2x80x99, although the sound field it produces is an approximation to the sound field that would be produced by a combination of point monopole and point dipole sources.
According to one aspect of the present invention, a sound reproduction system comprises loudspeaker means, and loudspeaker drive means for driving the loudspeaker means in response to signals from at least one sound channel, the loudspeaker means comprising a closely-spaced pair of loudspeakers, defining with the listener an included angle of between 6xc2x0 and 20xc2x0, inclusive, the loudspeaker drive means comprising filter means.
The included angle may be between 8xc2x0 and 12xc2x0 inclusive, but is preferably substantially 10xc2x0.
The filter means may comprise or incorporate one or more of cross-talk cancellation means, least mean squares approximation, virtual source imaging means, head related transfer means, frequency regularisation means and modelling delay means.
The loudspeaker pair may be contiguous, but preferably the spacing between the centres of the loudspeakers is no more than about 45 cms.
The system is preferably designed such that the optimal position for listening is at a head position between 0.2 meters and 4.0 meters from the loudspeakers, and preferably about 2.0 meters from said loudspeakers. Alternatively, at a head position between 0.2 meters and 1.0 meters from the loudspeakers.
The loudspeaker centres may be disposed substantially parallel to each other, or disposed so that the axes of their centres are inclined to each other, in a convergent manner.
The loudspeakers may be housed in a single cabinet.
The loudspeaker drive means preferably comprise digital filter means.
According to a second aspect of the present invention, a stereo sound reproduction system comprises a closely-spaced pair of loudspeakers, defining with a listener an included angle of between 6xc2x0 and 20xc2x0 inclusive, a single cabinet housing the two loudspeakers, loudspeaker drive means in the form of filter means designed using a representation of the HRTF (head related transfer function) of a listener, and means for inputting loudspeaker drive signals to said filter means.
According to a third aspect of the present invention, a stereo sound reproduction system comprises a closely-spaced pair of loudspeakers, defining with the listener an included angle of between 6xc2x0 and 20xc2x0 inclusive, and converging at a point between 0.2 meters and 4.0 meters from said loudspeakers, the loudspeakers being disposed within a single cabinet.
In accordance with a fourth aspect, the present invention may also be implemented by creating sound recordings that can be subsequently played through a closely-spaced pair of loudspeakers using xe2x80x98conventionalxe2x80x99 stereo amplifiers, filter means being employed in creating the sound recordings, thereby avoiding the need to provide a filter means at the input to the speakers.
The filter means that is used to create the recordings preferably have the same characteristics as the filter means employed in the systems in accordance with the first and second aspects of the invention.
The fourth aspect of the invention enables the production from conventional stereo recordings of further recordings, using said filter means as aforesaid, which further recordings can be used to provide loudspeaker inputs to a pair of closely-spaced loudspeakers, preferably disposed within a single cabinet.
Thus it will be appreciated that the filter means is used in creating the further recordings, and the user may use a substantially conventional amplifier system without needing himself to provide the filter means.
A sixth aspect of the invention is a recording of sound which has been created by subjecting a stereo or multi-channel recording signal to a filter means which is capable of being used in the system in accordance with the first or second aspects of the invention.